113 lines
3.7 KiB
NASM
113 lines
3.7 KiB
NASM
|
; From Ed Beroset - ISBN validation
|
||
|
|
; > Anyone know how to validate the ISBN numbers from books?
|
||
|
|
|
||
|
|
; isbnchek.asm
|
||
|
|
comment ^
|
||
|
|
|
||
|
|
This file contains a C-callable routine which calculates the
|
||
|
|
check digit (tenth digit) of an ISBN and returns the ASCII
|
||
|
|
representation of that digit.
|
||
|
|
|
||
|
|
This code was written for Borland's TASM
|
||
|
|
and may be assembled with the following command:
|
||
|
|
|
||
|
|
tasm /m2 isbnchek.asm
|
||
|
|
|
||
|
|
^
|
||
|
|
.MODEL small
|
||
|
|
|
||
|
|
public isbncheck
|
||
|
|
|
||
|
|
.CODE
|
||
|
|
;/***************************************************************************
|
||
|
|
;
|
||
|
|
; Name:
|
||
|
|
; isbncheck
|
||
|
|
;
|
||
|
|
; Purpose:
|
||
|
|
; Calculates the check digit for a ten digit ISBN, converts that
|
||
|
|
; digit to its ASCII representation and returns that answer.
|
||
|
|
;
|
||
|
|
; Algorithm:
|
||
|
|
; An ISBN consists of nine digits plus a validation digit.
|
||
|
|
; Number the digits from left to right as d1, d2, ... d9, with
|
||
|
|
; d10 being the validation digit. The calculation is then
|
||
|
|
;
|
||
|
|
; d10 = (1(d1) + 2(d2) + 3(d3) + ... + i(di) + ... + 9(d9))%11
|
||
|
|
;
|
||
|
|
; or the weighted sum of each digit mod eleven.
|
||
|
|
;
|
||
|
|
; In our assembly language implementation, we simulate the
|
||
|
|
; multiplications by looping through and summing as in the
|
||
|
|
; following psuedocode:
|
||
|
|
;
|
||
|
|
; sum = 0
|
||
|
|
; for i=1 to 9
|
||
|
|
; {
|
||
|
|
; for j=i to 9
|
||
|
|
; {
|
||
|
|
; sum = sum + isbn[j]
|
||
|
|
; }
|
||
|
|
; }
|
||
|
|
;
|
||
|
|
; Entry:
|
||
|
|
;
|
||
|
|
; isbn = a nine digit ASCII string containing the ISBN
|
||
|
|
; (with or without the check digit which is not used here)
|
||
|
|
;
|
||
|
|
; Register usage within the routine:
|
||
|
|
;
|
||
|
|
; AL = current ISBN digit
|
||
|
|
; AH = sum of digits so far
|
||
|
|
; BX = start pointer into ISBN for each outer loop
|
||
|
|
; CX = digit counter (inner loop)
|
||
|
|
; DX = start value for digit counter
|
||
|
|
; SI = points to current ISBN digit
|
||
|
|
;
|
||
|
|
; Exit:
|
||
|
|
;
|
||
|
|
; AX = ASCII representation of calculated check digit
|
||
|
|
;
|
||
|
|
; Trashed:
|
||
|
|
; none
|
||
|
|
;
|
||
|
|
;***************************************************************************/
|
||
|
|
isbncheck proc C isbn:ptr byte
|
||
|
|
push bx
|
||
|
|
push cx
|
||
|
|
push dx
|
||
|
|
push si
|
||
|
|
mov bx,[isbn] ;
|
||
|
|
mov dx,9 ; number of digits in raw ISBN
|
||
|
|
xor ax,ax ; clear out our total
|
||
|
|
cld ; count up
|
||
|
|
@@bigloop: ;
|
||
|
|
mov si,bx ; point to a digit in the ISBN
|
||
|
|
mov cx,dx ; get digit count in CX
|
||
|
|
@@AddEmUp: ;
|
||
|
|
lodsb ; fetch digit into AL
|
||
|
|
and al,0fh ; convert from ASCII
|
||
|
|
add ah,al ; add it to our total in AH
|
||
|
|
loop @@AddEmUp ; do all digits
|
||
|
|
inc bx ; and advance the digit pointer
|
||
|
|
dec dx ; now decrement digit count
|
||
|
|
jnz @@bigloop ; keep going if digits left
|
||
|
|
mov al,ah ; move sum into al
|
||
|
|
xor ah,ah ; clear out high half
|
||
|
|
mov cl,11 ; we'll be doing a mod 11 operation
|
||
|
|
div cl ; ah = sum mod 11
|
||
|
|
mov al,ah ; move calculated check digit to AL
|
||
|
|
xor ah,ah ; clear out high half
|
||
|
|
or al,30h ; convert to ASCII digit
|
||
|
|
cmp al,3Ah ;
|
||
|
|
jnz NotTen ;
|
||
|
|
mov al,'X' ;
|
||
|
|
NotTen: ;
|
||
|
|
pop si
|
||
|
|
pop dx
|
||
|
|
pop cx
|
||
|
|
pop bx
|
||
|
|
ret ; return
|
||
|
|
isbncheck endp
|
||
|
|
|
||
|
|
END
|